Meiosis

1. Meiosis Cell division – ‘reduction division’ Production of sex cells – gametes

2. Why do we need meiosis? • Meiosis is necessary to halve the number of chromosomes going into the sex cells • Making cells from diploid (2n) organisms into haploid (1N) sex cells – egg and sperm

3. …The Birds and the Bees • Occurs in humans and most other animals • Reproduce sexually. • This means that two cells fuse to make a new “baby” organism. • These specialized cells are called gametesor sex cells • Sperm and egg cells are human gametes • Happens in the testes in males and the ovaries in females

4. The Goal is to Make Haploid Sex Cells 1 Egg & 3 Polar Bodies 4 Sperm

5. What would happen if twocells with 46 chromosomes fused? 46 chromosomes + 46 chromosomes 92 chromosomes…. Uhhhhh ohhhhhh…

6. Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

7. Meiosis I • This is the stage where the chromosome number is halved Includes 4 stages: – Prophase 1 – Metaphase 1 – Anaphase 1 – Telophase 1

8. Prophase 1 Prophase 1 is similar in some ways to prophase in mitosis. • – Chromosomes condense • – Spindle fibers appear • – Nucleus and nucleolus disappear

9. Unlike prophase in mitosis… • Homologous chromosomes line up next to each other forming tetrads. • This process is called synapsis

10. Homologous Chromosomes • Human have 23 PAIRS of chromosomes. Each pair are called homologous chromosomes and you got one of the pair from mom and one from dad.

11. When homologous chromosomes hang out so close to each other, they sometimes swap parts!! • This is called “crossing over”

12. Synapsis and Crossing Over in Action

13. Metaphase 1 • Tetrads line up along the equator of the cell

14. Anaphase 1 • The spindle fibers pull apart the homologous chromosomes.

16. Telophase 1 • Two daughter cells are formed but they are no longer identical. • WHY???

17. THE BIG PICTURE

18. Meiosis 2 • Meiosis 2 is similar to mitosis. • The 2 cells formed from Meiosis 1 come in and are divided again. • This results in 4 cells each with half the number of chromosomes as the original starting cell.

19. Meiosis 2 • Prophase 2: spindle reforms and chromosomes move toward the metaphase plate • Metaphase 2: sister chromatids lined up on the metaphase plate • Anaphase 2: sister chromatids are separated and pulled toward opposite poles of the cell • Telophase 2 and Cytokinesis: nuclei form at either pole and each cell is finally divided into two identical daughter cells

20. Meiosis 2

21. Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

23. Meiosis – division error Chromosome pair

24. Meiosis error - fertilization Should the gamete with the chromosome pair be fertilised then the offspring will not be ‘normal’. In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

25. 21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?

26. Other Genetic Diseases Caused by problems with Meiosis • Klinefelter’s Syndrome – XXY instead of XX or XY. • Turner’s Syndrome – XO instead of XX or XY

27. Let’s Compare Meiosis with Mitosis

28. Division occuring in non-sex cells (blood, skin, ligament, etc) One set of divisions resulting in 2 cells Cells start out with 46 chromosomes and end up with 46 chromosomes and are identical to the original. Division producing sex cells (egg and sperm) Two sets of divisions resulting in 4 cells Cells start out with 46 chromosomes and end up with 23 chromosomes Mitosis vs Meiosis